The present invention relates to a driver for transmitting a digital differential signal, more particularly, it relates to a driver which can remove a common mode signal reflection which occurs in a transmission line and produces errors in signal transmission.
A digital differential signal is used for signal transmission in systems such as a local area network (LAN) system, and is used for protecting a signal being transmitted through a transmission line, such as a twisted wire pair, from electric noise which is received from outside the transmission line. As shown in FIG. 1, a transmission system for the digital differential signal consists of a driver 10, a transmission line X-X, and a transformer 12. Input digital differential signal D-D drives the driver 10 producing digital differential signal outputs on the transmission line X-X. The transmission line X-X carries the driven digital differential signal toward a receiving side where the transmission line X-X is terminated by the transformer 12. When standard No. 802.3 of the "Institute of Electrical and Electronics Engineers" (IEEE) is applied to the digital differential signal as is typical for the LAN system, the driver 10 in FIG. 1 is included in a "Data Terminal Equipment" (DTE), the transformer 12 is included in a "Medium Attachment Unit" (MAU), and the transmission line X-X corresponds to a "data out line" of an "Attachment Unit Interface" (AUI).
The digital differential signal output has two relevant voltages, a differential voltage and a common voltage. When V.sub.X is the instantaneous voltage on line X and V.sub.X that on line X, the differential voltage V.sub.diff is defined as: EQU V.sub.diff =V.sub.X -V.sub.X ( 1)
and the common voltage V.sub.comm is defined as: ##EQU1##
In the LAN system, data to be transmitted is encapsulated or grouped, coded, and transmitted as a packet, so the transmission line is either transmitting the packet or in a quiescent state. The time interval during actual transmission is called "packet time" and while the quiescent period is called "idle time". Data is transmitted only during the packet time. During the packet time, the X and X line voltages differentially change to a high (H) or a low (L) levels respectively in accordance with the data to be transmitted. The differential voltage levels of lines X and X do not change during the idle time because there is no signal or data to be transmitted. Furthermore, because the transmission line is terminated by the transformer 12, direct current cannot be allowed to flow through the transformer 12 to avoid saturation of the magnetic core therein. The driver 10 of the prior art will be discussed first with respect to the circuit diagram in FIG. 2 and related waveforms in FIG. 3.
The driver of the prior art, as illustrated in FIG. 2, consists substantially of emitter coupled logic (ECL) circuits to provide high speed operation. The transistors in transistor pairs Q1-Q2, Q5-Q6, and Q7-Q8 in FIG. 2 are matched and formed the ECL circuits along with their common emitter loads Q3, R2 and Q9, R6 which function as constant current sources. Each ECL circuit operates as a switching circuit, for example, in the ECL circuit Q1-Q2, Q2 is cut off when Q1 is on and vice versa because common emitter load Q3 and R2 keeps the current constant. A base voltage V.sub.cs, for example, 1.2 volts, is applied to Q3 is for proper bias.
Switching circuit Q5-Q6 acts as a driving circuit while transistors Q10 and Q11 are emitter followers which produce the digital differential signal output after receiving a differential output from the driving circuit at their bases. Output terminals T3 and T4 are connected to transmission lines X and X respectively. These circuits comprise a main portion of a circuit for a driver. Added to the main portion of the driver circuit are a packet time detecting circuit and a drive controller. Switching circuit Q1-Q2, transistor Q4, capacitor C, diode D1, and resistors R1 and R3 form the packet time detecting circuit which detects successive periods of packet time and allows the differential voltage on the transmission lines X-X to be gradually shifted to zero when the transmission line changes to the idle time. Switching circuit Q7-Q8 forms the drive controller which controls the driving circuit Q5-Q6 to produce a differential coded data signal during the packet time and to gradually produce a zero differential voltage during the idle time which becomes the initial voltage at the beginning of each packet time.
Details of the operation of the prior art driver circuit will be further explained for a case where "Manchester Coded Data" is the packet data coded according to IEEE Standard No. 802.3, using the waveforms in FIG. 3. In the packet according to the standard, the level of data (transmission line) during the idle time is "1" (D=H) and coded data during the packet time appears as a data train having at least an edge in one bit-time. FIGS. 3(a) and 3(b) respectively illustrate voltages of input signals D at T1 and D at T2 that are applied to switching circuits Q1-Q2 and Q5-Q6; FIGS. 3(c) and 3(d) respectively illustrate voltages of output signals at T3 and T4; FIG. 3(e) illustrates an output differential signal between T3 and T4; FIG. 3(f) illustrates a common voltage between the transmission line X-X and ground; and FIG. 3(g) illustrates periods in which symbol PT indicates packet time, ID1 and ID2 respectively indicate idle time just before and after PT.
During the period ID1, an idle signal (D=H, D=L) is applied to T1 and T2, so the driver is in a steady state as follows.
(1-1) As shown in parts a1 and b1 in FIGS. 3(a) and 3(b), the input voltage applied to the base of transistor Q1 is higher than input voltage applied to a base of transistor Q2, so Q1 is ON and Q2 is OFF.
(1-2) Since transistor Q1 is ON, a constant current from a constant current source consisting of Q3 and R2 flows through R1, this produces a voltage drop, based on the amount of constant current and the value of R1, at the collector of Q1. Since Q4, D1 and R3 form an emitter follower circuit, the voltage at the emitter of Q4 becomes V.sub.OL which is lower than the Q1 collector voltage by as much as V.sub.BE (base-emitter forward voltage 0.8 volt), and a voltage difference of V.sub.CC -V.sub.OL is applied to capacitor C.
(1-3) By adjusting the base voltage (reference voltage V.sub.r, for example, to 3 volts) of Q8 properly, base voltage of Q7 can be set lower than the reference voltage V.sub.r and Q7 will be OFF. The drive controller Q7-Q8 cuts off the driving circuit Q5-Q6. This causes the base voltages of emitter followers Q10 and Q11 to increase to V.sub.cc, which is, for example, 5 volts.
(1-4) The highest voltage level V.sub.OH appears on both output terminals T3 and T4; c1 and d1 in FIGS. 3(c) and 3(d), respectively, illustrate this situation. The differential voltage between X and X during period ID1 is zero as shown by e1 in FIG. 3(e). As a result, no direct current flows in the transformer 12 in FIG. 1, so core saturation in the transformer 12 is avoided.
When the period changes from ID1 to PT, an input digital differential signal is applied to terminals T1 and T2 as a pair of waveforms as shown by a3 and b3 in FIGS. 3(a) and 3(b), respectively. The pair of waveforms is standardized according to IEEE Standard No. 802.3. When the driver receives such a digital differential signal, the state of the driver changes as follows.
(2-1) Transistors Q1 and Q2 begin to be switched ON and OFF by the input digital differential signal. Q1 is switched off when initial negative pulse arrives at its base, transistor Q4 outputs a high level, which causes capacitor C to be quickly discharged, and then the base voltage of Q7 in the drive controller Q7-Q8 increases rapidly. Even though the next positive pulse cuts off Q4, the base voltage of Q7 is held high because the circuit including C, D1 and R3 has a large time constant as compared to a bit time that is, a next coded datum of L level is applied to the base of Q1 before the voltage at point A in FIG. 2 decreases. This state is maintained as long as the digital differential signal continues to be received, in other words, during the period of PT. This allows the above circuit to serve as a packet time detector.
(2-2) As Q7 is switched on, the voltage at the emitter of Q8 rises and Q8 is switched off. The drive controller Q7-Q8 controls the driving circuit Q5-Q6 to produce a digital differential signal from the collectors of Q5 and Q6 corresponding to the input differential signal applied to their bases.
(2-3) The emitter followers Q10 and Q11 produce the digital differential signal output as shown by c2 and d2 in FIGS. 3(c) and 3(d), respectively, on the transmission lines X-X. Reference symbol e2 in FIG. 3(e) indicates the differential voltage between the lines X and X during transmission.
When the period changes from PT to ID2, the state of the driver changes as follows.
(3-1) A voltage is applied to the base of Q1 to switch Q1 on, Q4 is cut off and C begins to be charged through D1 and R3 by the power source V.sub.cc. The base voltage of Q7 gradually decreases and the voltage at point A decreases and remains at the low level.
(3-2) The driving circuit Q5-Q6 tends to return to the same state as in ID1 discussed previously, however before returning to ID1, the following transient phenomena occurs in the circuit. As the Q7 base voltage gradually decreases, its collector current is gradually decreased, which affects the driving circuit Q5-Q6. Nothing happens to Q6 because it has been switched off, however, Q5 is affected because Q5 has been switched on. When the period changes to ID2 from PT at the beginning, the voltage at point A does not decrease at once, and the condition of Q7 being ON and Q8 being OFF is maintained because the base voltage of Q7 is sufficiently higher than that of Q8. After a while, when the voltage at point A decreases so that base voltages of Q7 and Q8 are equal, the current flow gradually changes from flowing in Q7 to flowing in Q8 which can be seen in FIGS. 3(c) and 3(d). In FIG. 3(c), c3 is caused by Q6 being OFF; the flat portion d3 in FIG. 3(d) is caused by Q5 being ON for a while; and the changing part d4 in FIG. 3(d) is caused by Q5 whose collector current gradually decreases. The phenomenon of the portion d4 affects the differential voltage between the transmission lines X and X, however, the influence appears gradually as shown by e4 in FIG. 3(e).
The differential voltage between the transmission lines consists of only the signals to be transmitted as shown by FIG. 3(e). As far as the FIG. 3(e) is concerned, no signal reflection occurs on the transmission line. However, a variation is concealed in the common voltage of the transmitting signal as shown by FIG. 3(f) which is a steep transient as illustrated by f2. This transient occurs due to the difference in mean voltage between the periods ID1 and PT. The difference can be calculated using the equation (2), as is shown by FIG. 3(f). The steep transient is transmitted by the transmission line X-X, and makes a round trip between driver 10 and transformer 12 in FIG. 1, because a reflection occurs due to a mismatched common mode impedance at the transformer. It is actually very hard to match common mode impedances perfectly and the reflection of the transient cannot be avoided. Another transient f3 in FIG. 3(f) is gentle, so no reflection occurs. When a reflection occurs, the reflected wave is superimposed on the transmitted digital differential signal and causes unexpected errors in the transmitted signals. This steep variation in the common voltage is a problem in the prior art devices.